The Tiananmen Square killings of 4 June 1989, 20 years ago, remain the most deadly events in the People's Republic of China since the death of Mao in 1976. Merely mentioning them can lead to arrest and detention. More than a dark shadow, the Tiananmen nightmare still hovers over the country.

Here are the essential details of what is officially still called "the incident" or "the events". On 15 April 1989, deposed party general secretary Hu Yaobang died. He had incurred the displeasure of senior leader Deng Xiaoping for being relaxed about dissent. But students, who liked him for his honest, country bumpkin ways, assembled in their thousands to mourn him in Tiananmen, the world's largest man-made space. The crowds grew ever larger, and on 26 April, the Communist Party newspaper People's Daily condemned the demonstrators for conspiring to destabilise China. This enraged the students, who from then on called for a retraction, greater government openness and less corruption. Real democracy was never demanded, but there were huge shouts for the end of party rule and the removal of the unpopular Premier, Li Peng, and even of Deng Xiaoping. At their height the demonstrators, no longer just students, numbered over one million. On the night of 3-4 June, the square was violently cleared and hundreds lay dead.

From single incidents to subsequent developments at the highest level of Chinese politics, one cannot exaggerate the importance of Tiananmen. Nor were the "events" confined to Beijing alone. There were hundreds of uprisings, from Mongolia, in the north-west, to the deep south. In addition to the numbers killed — and there has never been an official figure since the government declared within days of the massacre that "not a shot was fired, not a person was killed" — thousands were imprisoned.

To this day, if the word Tiananmen appears on the Chinese internet, whoever has used or accessed it can expect a knock on the door and may join the dozens of Tiananmen activists still in China's jails and labour camps. Such prisoners used to be convicted of "counter-revolution". Now they are simply "criminals".